DE69713053T2 - Image forming apparatus - Google Patents

Description

BACKGROUND OF THE INVENTION

The present invention relates generally to image forming apparatuses and, more particularly, to an image forming apparatus provided with a plurality of process units that record images on a recording medium such as continuous recording paper using electrophotography technology.

When images are recorded on both sides of the recording paper using a single process unit, the recording paper is turned over after the image is recorded on a first side of the recording paper, and thereafter the image is recorded on a second side of the recording paper. For this reason, there are limits to increasing the image recording speed, and a method of using two process units for respectively recording the images on the first and second sides of the recording paper has been proposed.

On the other hand, when a color image is recorded, there is a method in which a single process unit is used. In this case, the single process unit is used to record images of each of the colors such as yellow, magenta, cyan and black on the recording paper successively in an overlapping manner, and there are also limits to increasing the image recording speed. Therefore, a method has been proposed to provide a plurality of process units and record the image of each color using one of the process units.

In a printer that uses electrophotography technology and records the image on the continuous recording paper, conventionally, if a printing operation (job) ends and during a predetermined time, a portion of the paper printed with the last line is stored within a paper transport path inside the printer. Therefore, the recording paper is manually or automatically transported to discharge the portion of the paper printed with the last line from the printer so that the recording paper can be separated along a perforation provided subsequent to the portion of the paper printed with the last line.

However, if the next job is started in this state, printing is started in a state where the amount of the recording paper that has been advanced to remove the print result of the previous job is before the first line of this next job. As a result, the amount of the recording paper that has been advanced is wasted and the use efficiency of the recording paper is poor.

Conventionally, therefore, a so-called back-feed is performed if a job ends and the next job does not occur during a predetermined time. By this back-feed, the recording paper is further fed if a job ends but the next job does not occur during the predetermined time and is returned by a predetermined amount before the next job starts. As a result, the use efficiency of the recording paper is improved. A photoconductive body of the process unit and the recording paper are separated from each other when the back-feed is performed.

In this context, US-A-4984029 discloses a roller for transporting a continuous recording medium in two directions to overlay images formed at contactable development stations.

Similarly, in the case of the printer which records images, for example, by the first and second process units successively, the last line to be recorded is located on a downstream side of the second process unit in a transport direction of the recording paper at a time when a job ends. Thus, if the next job is started in this state, the portion of the paper between the first and second process units is wasted. For this reason, a method has been proposed to carry out the above-described back transport also in such a case.

However, in the printer which records the images by the first and second process units successively, if a single fixing unit is used in order to simplify the printer construction, this fixing unit is provided on the downstream side of the second process unit which is located most downstream along the transport direction of the recording paper. Accordingly, when the above-described reverse transport is carried out, the image recorded on the recording paper by the previous job is not yet fixed. Therefore, when the photoconductive body of each process unit and the recording paper come into contact at the start of the next job, there has been a problem that the image which is not yet fixed may be affected by the shock at the contact between the photoconductive body and the recording paper. If the image that is not yet fixed is affected, the image recording quality is extremely deteriorated and it becomes necessary to execute the two jobs again, which causes another problem that the performance of the printer is extremely deteriorated.

SUMMARY OF THE INVENTION

Therefore, it is a general object of the present invention to provide a new and useful image forming apparatus in which the above-described problems are eliminated.

Another and more specific object of the present invention is to provide an image forming apparatus capable of recording images with high quality and high speed.

Still another object of the present invention is to provide an image forming apparatus comprising a plurality of process units that contact a continuous recording medium and transfer images to the recording medium, a separating and contacting means for controlling at least one of the recording medium and each of the process units to assume a separating state in which the recording medium and the process unit are separated from each other and a contacting state in which the recording medium and the process unit are in contact with each other, and a control means for controlling the separating and contacting means at a position where the process unit faces a non-printing zone on the recording medium to place the recording medium and the process unit in the contacting state. According to the image forming apparatus of the present invention, it is possible to prevent the image not yet fixed on the recording medium from being affected when the reverse transport is carried out with respect to the recording medium. As a result, it is possible to prevent deterioration of the image recording quality and to carry out the image recording at high speed.

Other objects and further features of the present invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a diagram showing the general construction of a first embodiment of an image forming apparatus according to the present invention;

Fig. 2 is a diagram showing an embodiment of a process unit;

Figs. 3A, 3B and 3C are diagrams for explaining the printing operation of the first embodiment, respectively;

Fig. 4 is a flowchart for explaining a separating and contacting mechanism control process of a controller;

Fig. 5 is a flowchart showing an embodiment of a process in the case where information for obtaining a detection result of a non-printing zone is input from outside a printer;

Fig. 6 is a flow chart showing an embodiment of a process in the case where information for obtaining the detection result of the non-pressure zone from a sensor is input;

Fig. 7 is a perspective view showing an arrangement of the sensor;

Fig. 8 is a flow chart showing an embodiment of a process for contacting a photoconductive body of the process unit and a recording paper after return transport;

Figs. 9A and 9B are diagrams showing the construction that separates and contacts the recording paper with respect to each processing unit, respectively;

Fig. 10 is a diagram showing the general construction of a process part of a second embodiment of the Image forming apparatus according to the present invention;

Fig. 11 is a diagram showing the general construction of an important part of the process unit of a third embodiment of the image forming apparatus according to the present invention;

Fig. 12 is a perspective view showing a guide roller of Fig. 11;

Fig. 13 is a diagram showing the general construction of an important part of a first modification of the third embodiment;

Fig. 14 is a plan view showing a guide roller part of Fig. 13;

Fig. 15 is a diagram showing the general construction of an important part of a second modification of the third embodiment;

Fig. 16 is a diagram showing the general construction of an important part of a third modification of the third embodiment; and

Fig. 17 is a diagram showing the general construction of the process part of a fourth embodiment of the image forming apparatus according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig. 1 is a diagram showing the general construction of a first embodiment of an image forming apparatus according to the present invention. In this embodiment, the present invention is applied to a printer using the electrophotography technique.

A printer 1 shown in Fig. 1 generally includes a power supply 2, a controller 3, a paper feeding part 4, a process part 5, a separating and contacting mechanism 6, a fixing unit 7, a paper separating part 8, a stacker 9, and a sensor 10. The controller 3 includes a central processing unit (CPU) or the like which controls the entire printing operation of the printer 1 by controlling the operations of the various parts within the printer 1, such as the paper feeding part 4, the process part 5, the separating and contacting mechanism 6, the fixing unit 7, and the paper separating part 8. The paper feeding part 4 receives a continuous recording paper 11 as a recording medium from a pre-processing mechanism (not shown) and feeds the recording paper 11 into the printer 1 by a known means. The pre-processing mechanism may be a so-called hopper, and this hopper may be provided within the printer 1. The recording paper 11 fed into the printer 1 is transported by a known transport means (not shown) in a transport direction indicated by arrows in Fig. 1 along a predetermined transport path 12 within the printer 1. It is not necessarily necessary to provide the sensor 10 as described later.

The process part 5 includes a plurality of process units that transfer images onto the recording paper 11 using the electrophotography technique under the control of the controller 3. The separation and contacting mechanism 6 includes a solenoid, an air pump or the like, and controls at least one of the recording paper 11 and each of the process units to assume a separation state in which the recording paper 11 and the process unit are separated from each other, and a contact state in which the recording paper 11 and the Process unit contact each other under the control of the controller 3. The controller 3 determines the timing at which the separating and contacting mechanism 6 is controlled with respect to the separating state and the contacting state, on the basis of an internal timer, an output signal of the sensor 10 and the like.

Under the control of the controller 3, the fixing unit 7 fixes the image transferred to the recording paper 11 by contacting the recording paper 11 or not. The paper separating part 8 feeds the recording paper 11 on which the image is recorded to a post-processing mechanism (not shown) or stacks the recording paper 11 on which the image is recorded on the stacker 9 inside the printer 1 under the control of the controller 3. Perforations are formed at predetermined intervals on the recording paper 11, and the recording paper 11 is successively stacked on the stacker 9 by being alternately folded in opposite directions along the perforations.

Fig. 2 is a diagram showing an embodiment of the process unit. In this embodiment, it is assumed for the sake of simplicity that the process part 5 includes two process units each having the construction shown in Fig. 2.

A process unit 20 generally includes, as shown in Fig. 2, a precharger 21, an exposure part 22, a developing part 23, a transfer part 24, an AC discharger 25, a cleaner part 26, a light emitting diode (LED) discharger 27, and a photoconductive body 28. In this embodiment, the photoconductive body 28 includes a photoconductive drum. At the time of printing, the photoconductive body 28 is rotated clockwise by a known means as indicated by an arrow, and the surface of the photoconductive body 28 is uniformly charged by the precharger 21. The exposure part 22 exposes a pattern corresponding to the image to be recorded on the recording paper 11 on the surface of the photoconductive body 28 to form an electrostatic latent image. This electrostatic latent image is developed into a toner image by the developing part 23.

On the other hand, the recording paper 11 is transported by a known means in a direction indicated by an arrow in Fig. 2. Therefore, when the photoconductive body 28 rotates, the toner image is transferred to the transported recording paper 11 by the transfer member 24. This toner image is fixed by the fixing unit shown in Fig. 1 provided in a subsequent stage.

The charge of the residual toner remaining on the surface of the photoconductive body 28 after the toner image is transferred to the recording paper 11 is eliminated by the AC discharger 25 and mechanically removed by a cleaning blade or a brush of the cleaning part 26. Thereafter, the LED discharger 27 returns the surface potential of the photoconductive body 28 to the initial level (for example, 0 V).

3A to 3C are diagrams for explaining the printing operation of the first embodiment, respectively. In FIGS. 3A to 3C, those parts which are the same as the corresponding parts of FIG. 2 are denoted by the same reference numerals, and in addition, parts of a first process unit 20-1 are denoted by the suffix "-1" and parts of a second process unit 20-2 are denoted by the suffix "-2", and a description thereof is omitted. For the sake of simplicity, FIGS. 3A to 3C only important parts of each of the process units 20-1 and 20-2.

Fig. 3A shows a stage where a first printing operation (job) JOB1 ends, a toner image TI is transferred to the recording paper 11 by the first and second process units 20-1 and 20-2, and under the control of the controller 3, the separating and contacting mechanism 6 controls at least one of the recording paper 11 and each of the process units 20-1 and 20-2 to assume the separation state in which the recording paper 11 and each of the process units 20-1 and 20-2 are separated from each other. When a next, the second printing operation (job) JOB2 is started in this state, the part of the recording paper 11 between the first and second process units 20-1 and 20-2 remains unprinted, and the use efficiency of the recording paper 11 is poor. Therefore, the return transport is performed as indicated by an arrow to guide the recording paper 11 in a direction opposite to the normal transport direction during printing. By this return transport, the recording paper 11 is returned to a position where a non-printing zone NP on the recording paper 11 faces the photoconductive body 28-1 and the transfer part 24-1 of the first process unit 20-1. Although the toner image TI transferred to the recording paper 11 by the first job JOB1 is not yet fixed, the photoconductive body 28-1 of the first process unit 20-1 and the surface of the recording paper 11 onto which the toner image TI was transferred are separated from each other, and the toner image TI is not affected by the return transport. The non-printing zone NP is a zone on the recording paper 11 onto which no image is printed, such as a section that is perforated.

The illustration of a transport means for transporting the recording paper 11 in the transport direction during normal recording and for transporting the recording paper 11 in a direction opposite to the transport direction during return transport is omitted. It is of course possible to use a known transport means to carry out such transport of the recording paper 11.

Fig. 3B shows a state when the return transport of the recording paper 11 is carried out as described above, and in order to start the second job JOB2, the separating and contacting mechanism 6, under the control of the controller 3, controls at least one of the recording paper 11 and the first process unit 20-1 to assume the contact state in which the recording paper 11 and the first process unit 20-1 contact each other. Since the photoconductive body 28-1 of the first process unit 20-1 comes into contact with the non-printing zone NP on the recording paper 11, the toner image TI is not affected upon contact. The image transfer of the second job JOB2 by the first process unit 20-1 is started in this state.

Fig. 3C shows a state when a predetermined time comes to an end from the start of the second job JOB2, and in order to start the image transfer by the second process unit, the separating and contacting mechanism 6, under the control of the controller 3, controls at least one of the recording paper 11 and the second process unit 20-2 to assume the contact state in which the recording paper 11 and the second process unit 20-2 contact each other. The image transfer of the second job JOB2 by the second process unit 20-2 is started in this state.

As described above, the controller 3 controls the separating and contacting mechanism 6 so that the process unit comes into contact with the recording paper 11 in an order starting from the process unit that transfers the image, that is, the process unit provided on the upstream side with respect to the conveying direction of the recording paper 11. Particularly, when the number of process units is large, when all the process units contact the recording paper 11 at the same time, the toner image not yet fixed on the recording paper 11 may be deteriorated due to the vibration of the recording paper 11 or the like caused by the shock at the contact. However, if the number of processing units is small or the vibration of the recording paper 11 or the like when the processing units and the recording paper 11 contact can be suppressed, all the processing units may contact the recording paper 11 at the same time, or the processing units may be divided into groups each including a plurality of processing units, and the processing units within the same group may contact the recording paper 11 at the same time.

Next, a description will be given of the control timing of the separating and contacting mechanism 6 by the controller 3 with reference to Fig. 4. Fig. 4 is a flow chart for explaining a separating and contacting mechanism control process of the controller 3. For the sake of simplicity, it is assumed that the jobs JOB1 and JOB2 shown in Figs. 3A to 3C are executed.

In Fig. 4, a job is executed at step S1 if the job is input to the printer 1 from an external host unit (not shown) or the like in the form of print information and control information. In this case, the job JOB1 is executed by the first and second process units 20-1 and 20-2. At step S2, it is determined whether or not the next job has been input. The process ends if the determination result at step S2 is NO, and the process goes to a routine or the like for feeding the recording paper 11 forward. Otherwise, if the determination result at step S2 is YES, it is determined at step S3 whether or not the previous job JOB1 has ended.

If the determination result in step S3 is YES, in step S4, the separating and contacting mechanism 6 is controlled and at least one of the recording paper 11 and each of the first and second process units 20-1 and 20-2 is controlled to assume the separation state in which the recording paper 11 and each of the first and second process units 20-1 and 20-2 are separated from each other. Specifically, the solenoid, air pump or the like of the separating and contacting mechanism 6 acts on parts supporting rotation shafts of the photoconductive bodies 28-1 and 28-2 of the first and second process units 20-1 and 20-2 to separate the photoconductive bodies 28-1 and 28-2 from the recording paper 11. At step S5, the transport means is controlled in this separation state to carry out the reverse transport and transport the recording paper 11 in the direction opposite to the transport direction during normal recording. At step S6, it is determined whether or not the photoconductive body 28-1 of the first process unit 20-1 has reached a position corresponding to the non-printing zone NP on the recording paper. 11 on the upstream side in the transport direction with respect to the last line of the toner image TI transferred to the recording paper 11 by the job JOB1. The process returns to step S5 if the determination result in step S6 is NO. The position of the recording paper 11 at which the photoconductive body 28-1 faces the non-printing zone NP on the recording paper 11 on the upstream side in the transport direction with respect to the last line of the toner image TI transferred to the recording paper 11 by the job JOB1 can be calculated using a detection result of the non-printing zone NP based on a distance between the first and second process units 20-1 and 20-2 or the like.

Otherwise, if the determination result at step S6 is YES, at step S7, the separating and contacting mechanism 6 is controlled and at least one of the recording paper 11 and the first process unit 20-1 is controlled to assume the contact state in which the recording paper 11 and the first process unit 20-1 are in contact with each other. In addition, at step S8, the next job JOB2 is executed by the first process unit 20-1. When the timing is reached to execute the job JOB2 by the second process unit 20-2, at step S9, the separating and contacting mechanism 6 is controlled and at least one of the recording paper 11 and the second process unit 20-2 is controlled to assume the contact state in which the recording paper 11 and the second process unit 20-2 are in contact with each other. Further, at step S10, the job JOB2 is executed by the second process unit 20-2, and the process returns to step S2.

Fig. 5 is a flow chart showing an embodiment of the process executed by step S6 in the case where information for obtaining the detection result of the non-printing zone NP is input from outside the printer 1. In this case, the sensor 10 shown in Fig. 1 may be omitted.

In Fig. 5, at step S601, it is determined whether or not paper information regarding the size or the like of the recording paper 11 has been input to the controller 3 through the control information. The paper information is input by the operator from the host unit or from an operation panel of the printer 1, for example. Alternatively, if a mechanism is provided to automatically judge the size or the like of the recording paper 11 when the recording paper 11 is placed in the printer 1, the paper information can be automatically input to the printer 1 from such a mechanism.

If the determination result in step S601 is YES, an internal timer of the controller 3 is started in step S602. Since the perforations of the recording paper 11 are provided periodically, in step S603, the position of the recording paper 11 at which the photoconductive body 28-1 faces the non-printing zone NP on the recording paper 11 on the upstream side in the transport direction with respect to the last line of the toner image TI transferred to the recording paper 11 by the job JOB1 is calculated, and whether or not the recording paper 11 has been transported back to the calculated position is determined. This calculated position is calculated by step S603 on the basis of a count value of the internal timer, the transport amount and the transport direction of the recording paper 11, the distance between the first and second processing units 20-1 and 20-2 and the like. If the determination result in step S603 is YES, the process proceeds to step S7 shown in Fig. 4. Otherwise, if the determination result in step S603 is YES, the process returns to step S5 shown in Fig. 4.

Fig. 6 is a flowchart showing an embodiment of the process executed by step S6 in the case where information for obtaining the detection result of the non-pressure zone NP from the sensor 10 is input.

In Fig. 6, at step S605, a toner mark TM is transferred to the recording paper 11 at a perforation portion by the first process unit 20-1 or the second process unit 20-2 in advance before the return transport is carried out. Since the perforations of the recording paper 11 are periodically provided, the timing at which the toner mark TM is transferred to the recording paper 11 can be obtained similarly to the case shown in Fig. 5 described above. At step S606, it is determined whether or not the output signal of the sensor 10 indicating the detection of the toner mark TM has been received. The arrangement of the sensor 10 is not limited to the inside of the process part 5, but it may be provided at any location on the downstream side in the transport direction with respect to the process unit which transfers the toner mark TM at the perforation portion onto the recording paper 11.

Fig. 7 is a perspective view showing the case where the toner mark TM is transferred to the recording paper 11 by the first process unit 20-1 and the sensor 10 is arranged at a position corresponding to the downstream side of the first processing unit 20-1 in the transport direction and on the upstream side of the second processing unit 20-2 in the transport direction. The sensor 10 is, for example, an optical reading means formed of a charge coupled device (CCD).

If the determination result in step S606 is YES, in step S607, the position of the recording paper 11 at which the photoconductive body 28-1 faces the non-printing zone NP on the recording paper 11 on the upstream side in the conveying direction with respect to the last line of the toner image TI transferred to the recording paper 11 by the job JOB1 is calculated based on the output signal of the sensor 10, and it is determined whether or not the recording paper 11 has been conveyed back to the calculated position. If the determination result in step S607 is YES, the process proceeds to step S7 shown in Fig. 4. On the other hand, if the determination result in step S607 is NO, the process returns to step S5 shown in Fig. 4.

Fig. 8 is a flow chart showing an embodiment of the process carried out by step S7. In Fig. 8, at step S701, the separating and contacting mechanism 6 is controlled so that the photoconductive body 28-1 of the first process unit 20-1 and the recording paper 11 assume the contact state. At step S702, the start timing of the image transfer by the second process unit 20-2 arranged on the downstream side in the transport direction is calculated based on the print information of the job JOB2. At step S703, it is determined whether or not the start timing of the image transfer by the second process unit 20-2 is reached.

If the determination result in step S703 is YES, in step S704, the separating and contacting mechanism 6 is controlled so that the photoconductive body 28-2 of the second process unit 20-2 and the recording paper 11 assume the contact state. Thereafter, the process returns to step S8 shown in Fig. 4.

When the recording paper is moved between the contact state and the separation state with respect to each processing unit, it is possible to employ a structure shown in Figs. 9A and 9B. Figs. 9A and 9B respectively show the structure that separates and contacts the recording paper with respect to each processing unit by driving a paper guide that guides the recording paper through the separation and contacting mechanism. In Figs. 9A and 9B, those parts that are substantially the same as the corresponding parts of Fig. 2 are denoted by the same reference numerals, and a description thereof is omitted.

Fig. 9A shows the case where the solenoid, the air pump or the like of the separating and contacting mechanism 6 is active, a paper guide 29 is pushed in a direction toward the photoconductive body 28 of the process unit 20 by the separating and contacting mechanism 6, and the photoconductive body 28 and the recording paper 11 are in the contact state. On the other hand, Fig. 9B shows the state where the separating and contacting mechanism 6 is inactive and the photoconductive body 28 and the recording paper 11 are in the separating state because the paper guide 29 is arranged at a recessed position.

For the sake of simplicity, this embodiment is described for the case where two processing units are provided, but similar control can be carried out in cases where more than three processing units are provided. It is possible to print in two colors if two process units are provided. For example, if four process units are provided using yellow toner, magenta toner, cyan toner and black toner, it is therefore possible to print in full color.

Further, this embodiment will be described for the case where each of the processing units transfers the images to the same side of the recording paper, but a plurality of processing units may be provided with respect to the first side and the second side of the recording paper. In this case, both-sided printing of images is possible. In addition, if a sufficient number of processing units are provided with respect to the first and second sides of the recording paper, it is possible to perform full-color both-sided printing.

When the image transfer to the first side of the recording paper is performed by the first process unit, the image transfer to the second side of the recording paper is performed by the second process unit, and the fixing of the images is carried out at the end, the toner image transferred to the first side of the recording paper by the first process unit contacts the transfer part of the second process unit as this toner image passes through the second process unit. The transfer part transfers the toner image to the recording paper using an electric field opposite to the polarity of the charge of the toner image formed on the surface of the photoconductive body. For this reason, in the case where the polarities of the charges of the toner images formed on the surfaces of the photoconductive bodies of the first and second process units are the same, the toner image that transferred to the first side of the recording paper by the first process unit and not yet fixed is attracted to the transfer part of the second process unit. As a result, depending on the amount of toner attracted to the transfer part of the second process unit, normal image transfer to the second side of the recording paper cannot be performed due to toner stains on the transfer part, thereby causing a printing defect. Next, a description will be given of a second embodiment of the image forming apparatus according to the present invention, in which such a printing defect can be prevented with certainty.

The general construction of the second embodiment of the image forming apparatus is basically the same as the construction of the first embodiment shown in Fig. 1, so that an illustration thereof is omitted. In this second embodiment, the process part has a construction shown in Fig. 10.

Fig. 10 is a diagram showing the general construction of the process part 5 of the second embodiment. In Fig. 10, those parts which are substantially the same as the corresponding parts of Figs. 2 and 3 are given the same reference numerals, and a description of them is omitted. In addition, in Fig. 10, a developing agent having a charge of negative polarity is indicated by a black round mark, a developing agent having a charge of positive polarity is indicated by a white round hatched mark, and a neutralized developing agent is indicated by a white round mark.

In Fig. 10, the surface of the first photoconductive body 28-1 is heated by the first charger 21-1 to which a DC voltage of, for example, -7 kV is applied. charged to about -600 V, and the electrostatic latent image is exposed by the first exposure part 22-1. A two-component developing agent formed of a carrier having a positive charge and a toner having a negative charge is provided in the first developing part 23-1, and the electrostatic latent image is visualized by the toner having a negative charge, whereby the negatively charged toner image is formed on the surface of the first photoconductive body 28-1. The first transfer part 24-1, to which a DC voltage of +500 V, for example, is applied, electrostatically attracts the negatively charged toner image and transfers the toner image to the first side of the recording paper 11.

Next, the negatively charged toner image on the first side of the recording paper 11 is charged by first and second reversal chargers 31 and 32 before it reaches the second process unit 20-2 on the downstream side in the transport direction of the recording paper 11. In other words, the negatively charged toner image (developing agent) on the first side of the recording paper 11 is charged with an opposite polarity by the first reversal charger 31 to which, for example, an AC voltage of 12 kV is applied, thereby neutralizing the negatively charged toner image on the first side. Further, the neutralized toner image (developing agent) on the first side of the recording paper 11 is charged with an opposite polarity by the second reversing charger 32 to which, for example, a DC voltage of +6 kV is applied, thereby making the neutralized toner image on the first side a positively charged toner image.

When the negatively charged toner image formed on the surface of the second photoconductive body 28-2 of the second process unit 20-2 is electrostatically attracted by the second transfer part 24-2 to which, for example, a DC voltage of +500 V is applied, and is transferred to the second side of the recording paper 11 similarly to the first process unit 20-1 described above, the positively charged toner image existing on the first side of the recording paper 11 is not attracted to the second transfer part 24-2 because the positively charged toner image has the same polarity as the positive electric field of the second transfer part 24-2 and is repelled by the second transfer part 24-2. For this reason, the toner (developing agent) transferred to the first side of the recording paper 11 and not yet fixed is surely prevented from adhering to the second transfer part 24-2 and causing the printing defect.

The polarities of the charged toner images transferred to the first and second sides of the recording paper 11 by the first and second process units 20-1 and 20-2 may of course be opposite to those described above.

In addition, instead of using the first and second reversing chargers 31 and 32, it is possible to provide a single reversing charger and similarly charge the toner image with the opposite polarity.

Furthermore, in the first and second processing units 20-1 and 20-2, and particularly in the first and second developing parts 23-1 and 23-2, it is possible to obtain effects similar to the above by using a developing agent having different charging characteristics, that is, having opposite polarities. In this case, the first and second reversing chargers 31 and 32 may be omitted, but on the other hand, not all Parts such as the development part may be used jointly for both the first and second process units 20-1 and 20-2.

On the upstream side of each processing unit in the transport direction of the recording paper, if the recording paper slips and a positional error or vibration is generated when the recording paper comes into contact with the photoconductive body, the recorded image is deteriorated and the image quality is deteriorated. Therefore, it is conceivable to provide a guide roller for guiding the recording paper to the upstream side of the processing unit in order to stably feed the recording paper to the processing unit. However, particularly in multi-color printing or double-sided printing, the guide roller comes into direct contact with the toner image which has already been transferred to the recording paper but is not yet fixed, and the recorded image is deteriorated in the direct contact. Therefore, a description will now be given of a third embodiment of the image forming apparatus according to the present invention, by which the recording paper can be stably fed to the process unit without causing such image deterioration.

The general construction of the third embodiment of the image forming apparatus is substantially the same as the construction of the first embodiment shown in Fig. 1, and an illustration thereof is omitted. In this third embodiment, the process part has a construction shown in Fig. 11.

Fig. 11 is a diagram showing the general construction of an important part of the process unit of the third embodiment. In Fig. 11, those parts which are the same as the corresponding parts of Fig. 10 are indicated by are given the same reference numerals, and a description thereof will be omitted. In Fig. 11, TI1 denotes a toner image which has been transferred to the first side of the recording paper 11 and is not yet fixed, and TI2 denotes a toner image formed on the surface of the second photoconductive body 28-2.

A guide roller 35 for guiding the recording paper 11 is provided on the upstream side of the second photoconductive body 28-2 of the second process unit 20-2, as shown in Fig. 11. This guide roller 35 is rotated at a speed vr that is approximately the same as a transport speed vp of the recording paper 11 transported by the transport means. The guide roller 35 can be rotated by a drive source (motor) 36, which may be the same as or different from a drive source that rotates the second photoconductive body 28-2. Therefore, on the upstream side of the second process unit 20-2 where the recording paper 11 comes into contact with the second photoconductive body 28-2, the recording paper 11 is stably fed without slipping. The guide roller 35 may be provided to be movable with respect to the second photoconductive body 28-2 by the separating and contacting mechanism 6 so that the guide roller 35 is separated from or comes into contact with the second photoconductive body 28-2 through the recording paper 11.

Fig. 12 is a perspective view showing the guide roller 35 from below. The guide roller 35 is formed of an aluminum core on which a silicon rubber layer is formed, and further the silicon rubber layer is coated with fluorocarbon resin or fluororubber. Both end portions 35a of the guide roller 35 have an outer diameter slightly larger than that of a central portion 35b. The end portions 35a of the guide roller 35 are in contact with the non-printed portions 11a of the recording paper 11, and the central portion 35b faces the printed portion on the recording paper 11. The friction coefficient of fluorocarbon plastic or fluororubber is, for example, 0.2, and Mueller strips, which are high friction members and have a friction coefficient of 1.0 to 2.0, are formed on the end portions 35a of the guide roller 35. For example, the outer diameter of the end portions 35a is 20.2 mm, and the outer diameter of the central portion 35b is 20.0 mm.

When the transport speed vp of the recording paper is, for example, 250 mm/s, it has been confirmed by experiments conducted by the present inventors that the error of the peripheral speed vr of the guide roller 35 with respect to the transport speed vp can be suppressed to the range of ±5% by rotating the guide roller 35 by the motor 36. Therefore, the recording paper 11 is stably fed to the second process unit 20-2, and the toner image TI1 transferred to the first side of the recording paper 11 and not yet fixed does not easily come into contact with the guide roller 35 due to the structure of the guide roller 35. As a result, the deterioration of the toner image TI1 not yet fixed is suppressed to an extremely small extent.

Instead of driving the guide roller 35 by the motor 36, it is of course possible to rotate the guide roller 35 by the second photoconductive body 28-2 to rotate therewith. In this case, through experiments carried out by the present inventors, It is confirmed that if the transport speed vp of the recording paper 11 is, for example, 250 mm/s and the same conditions as described above exist, the error of the peripheral speed vr of the guide roller 35 with respect to the transport speed vp can be suppressed to the range of ±10% by rotating the guide roller 35 by the second photoconductive body 28-2 to rotate therewith.

The end portion 35a, which has the diameter slightly larger than that of the central portion 35b, needs to be provided only at one end of the guide roller 35.

Fig. 13 is a diagram showing the general construction of an important part of a first modification of the third embodiment. In addition, Fig. 14 is a plan view showing a guide roller part of Fig. 13. In Figs. 13 and 14, those parts which are the same as the corresponding parts of Figs. 11 and 12 are denoted by the same reference numerals, and a description thereof is omitted.

In this modification, another guide roller 37 is provided between the guide roller 35 and the photoconductive body 28-2. A belt 39 is provided around a disk 35-1 arranged at one end of the guide roller 35 and a disk 37-1 arranged at one end of the guide roller 37. In addition, a roller 40 presses both end portions 35a of the guide roller 35 through the recording paper 11. The guide rollers 35 and 37 and the roller 40 may be rotated by rotating the guide roller 37 through the second photoconductive body 28-2 to rotate therewith, or at least one of the guide rollers 35 and 37 and the roller 40 may be driven by a drive source such as the motor 36. Furthermore, the outer peripheral surface of the guide roller 37 may have the same outer diameter, but preferably the guide roller 37 has the same construction as the guide roller 35.

According to this modification, it was confirmed through experiments conducted by the present inventors that it is possible to obtain effects similar to those achievable in the third embodiment under the same conditions as described above.

Fig. 15 is a diagram showing the general construction of an important part of a second modification of the third embodiment. In Fig. 15, those parts that are the same as the corresponding parts of Figs. 11 and 13 are designated by the same reference numerals, and a description thereof is omitted.

In this modification, a cleaning roller 43 made of felt material is provided with respect to the guide roller 35 of the third embodiment or the guide roller 37 of the first modification of the third embodiment. This cleaning roller 43 is designed to be rotated by the guide roller 35 or 37 so as to rotate therewith. Therefore, even if the toner not yet fixed on the recording paper 11 adheres a little to the guide roller 35 or 37, this toner is removed by the cleaning roller 43. Therefore, it is possible to surely prevent the toner image not yet fixed from being affected by the guide roller 35 or 37.

According to this modification, it was confirmed by experiments conducted by the present inventors under the same conditions as described above that if the transport speed vp of the recording paper 11 is, for example, 250 mm/s, it is possible to To suppress the error of the peripheral speed vr of the guide roller 35 or 37 with respect to the conveying speed vp to the range of ±8% even when the cleaning roller 43 is rotated by the guide roller 35 or 37 which is driven by the motor 36 to rotate with the guide roller 35 or 37.

Fig. 16 is a diagram showing the general construction of an important part of a third modification of the third embodiment. In Fig. 16, those parts that are the same as the corresponding parts of Figs. 11 and 13 are designated by the same reference numerals, and a description thereof is omitted.

In this modification, the recording paper 11 is moved by, for example, the separating and contacting mechanism 6 to separate from and contact the second photoconductive body 28-2. The guide roller 35 or 37 is arranged so that the recording paper 11 is wound around the guide roller 35 or 37 in an angular range of 10° or more in the contact state in which the recording paper 11 and the second photoconductive body 28-2 contact each other. Therefore, it is possible to prevent the recording paper 11 from rubbing and slipping when the recording paper 11 and the guide roller 35 or 37 are separated and contacted each other, thereby more securely preventing the toner image which is not yet fixed from being affected. On the other hand, in the case where the second photoconductive body 28-2 is moved by the separating and contacting mechanism 6 to separate from and contact the recording paper 11, the guide roller 35 or 37 may be arranged so that the recording paper 11 is in the contact state in which the recording paper 11 and the second photoconductive body 28-2 contact each other, is wound around the guide roller 35 or 37 over an angular range of 10º or more.

When a plurality of process units are used, such as in multi-color printing, the process units may have the same construction if the process units transfer the images with respect to the same side of the recording paper. Essentially, it is possible to cope with the multi-color printing simply by using different developing agents in the developing parts of the process units. For this reason, it is possible to reduce the number of kinds of parts and minimize the cost of the printer as a whole. However, when the double-sided printing is carried out using a plurality of process units, for example, two process units are provided, the two process units transfer the images with respect to mutually different sides of the recording paper, and in this case, the constructions of the two process units differ.

Even in the above-described case where the structures of the two process units are different, parts including the charger, the exposure part, the transfer part, the cleaner part, the discharger part and the photoconductive body need only be arranged symmetrically with respect to the transport path of the recording paper between the two process units. Therefore, the same parts can be used for the two process units with respect to such parts. However, with respect to the developing part, the direction of rotation of the developing roller within the developing part is opposite to each other in the two process units, and the same developing part cannot be used for the two process units. For this reason, it is necessary to arrange the developing part with respect to each of the two process units. Next, a description will be given of an embodiment in which the same development part can be used in two process units.

Fig. 17 is a diagram showing the general construction of the process part of a fourth embodiment of the image forming apparatus according to the present invention. In Fig. 17, those parts which are the same as the corresponding parts of Fig. 10 are denoted by the same reference numerals, and a description thereof is omitted.

In Fig. 17, the developing parts 23-1 and 23-2 themselves have known structures which are the same, including developing rollers 23A and 23B respectively, and a description regarding the structure of the developing parts 23-1 and 23-2 themselves will be omitted. This embodiment is characterized by setting the rotation directions of the developing rollers 23A and 23B with respect to the rotation directions of the corresponding photoconductive bodies 28-1 and 28-2. Specifically, in the first process unit 20-1, the photoconductive body 28-1 and the developing roller 23A both rotate clockwise. On the other hand, in the second process unit 20-2, the photoconductive body 28-2 rotates counterclockwise and the developing roller 23B rotates clockwise, that is, the photoconductive body 28-2 and the developing roller 23B rotate in opposite directions to each other. Accordingly, the first process unit 20-1 develops the toner image on the photoconductive body 28-1 using the so-called counter-rotation development in which the photoconductive body 28-1 and the developing roller 23A rotate in opposite directions to each other at the contact point, and the second process unit 20-2 develops the toner image on the photoconductive body 28-2 using the so-called co-rotation development in which the photoconductive body 28-2 and the developing roller 23B rotate at the contact point with each other. According to this embodiment, therefore, the parts constituting the process units can be used in common for both the process units 20-1 and 20-2 including the developing part. Therefore, it is possible to reduce the number of kinds of parts and minimize the cost of the printer as a whole.

The developing power when developing the toner image using counter-rotation development and when developing the toner image using co-rotation development differs slightly. Therefore, in order to make the quality of the image recording approximately the same with respect to both sides of the recording paper 11, at least one of the following methods should be used.

According to a first method, the rotation speeds of the developing rollers 23A and 23B within the developing parts 23-1 and 23-2 of the first and second process units 20-1 and 20-2 are set to different values. For example, if the outer diameters of the photoconductive bodies 28-1 and 28-2 are 100 mm and their peripheral speeds are 300 mm/s and the outer diameters of the developing rollers 23A and 23B are 40 mm, the peripheral speed of the developing roller 23A is set to 600 mm/s and the peripheral speed of the developing roller 23B is set to 700 mm/s. As a result, it is possible that the developing ability of the developing parts 23-1 and 23-2 becomes approximately the same.

According to a second method, the toner concentration used in the developing part 23-2 within the second process unit 20-2, in which the co-rotation development is used, is higher than the toner concentration used in the developing part 23-1 within the first process unit 20-1 to which the counter-rotation development is applied. For example, the toner concentration used in the developing part 23-1 is set to 4%, and the toner concentration used in the developing part 23-2 is set to 5%. As a result, it is possible that the developing ability of the developing parts 23-1 and 23-2 is approximately the same.

According to a third method, the carrier grain diameter used in the part 23-2 within the second process unit 20-2, in which the co-rotation development is used, is set smaller than the carrier grain diameter used in the developing part 23-1 within the first process unit 20-1, in which the counter-rotation development is used. For example, the carrier grain diameter used in the developing part 23-1 is set to 80 µm, and the carrier grain diameter used in the developing part 23-2 is set to 60.5 µm. As a result, it is possible that the developing power of the developing parts 23-1 and 23-2 is approximately the same.

According to a fourth method, the development bias voltage used in the development part 23-2 within the second process unit 20-2, in which the co-rotation development is used, is set higher than the development bias voltage used in the development part 23-1 within the first process unit 20-1, in which the counter-rotation development is used. For example, the development bias voltage used in the development part 23-1 is set to 300 V, and the development bias voltage used in the development part 23-2 is set to 350 V. As a result, it is possible that the development capacity of the development parts 23-1 and 23-2 is approximately the same.

Of course, it is possible to use the co-rotation development in the first process unit 20-1 and the counter-rotation development in the second process unit 20-2.

In order to facilitate maintenance of the printer, it is additionally desirable to design the process units so that the parts of each process unit that need to be maintained face the same side of the printer. By taking such measures, maintenance of the printer is facilitated and it is possible to reduce considerably the time required to repair or replace parts of the printer.

Furthermore, the plurality of processing units in Fig. 1 may be arranged vertically, arranged horizontally depending on the transport path, or arranged in combination with one or more processing units being arranged vertically and one or more processing units being arranged horizontally. Of course, the processing units may be arranged obliquely to the vertical direction depending on the arrangement of the transport path and the various parts within the image forming apparatus.

Furthermore, the present invention is not limited to these embodiments, but various changes and modifications can be made without departing from the scope of the present invention.

Claims (14)

1. An image forming apparatus having a plurality of process units (20) which contact a continuous recording medium and transfer images to the recording medium, characterized in that there are provided: a separating and contacting means (6) for controlling at least one of the recording medium and each of the process units to assume a separating state in which the recording medium and the process unit are separated from each other and a contacting state in which the recording medium and the process unit contact each other; and a control means (3) for controlling the separating and contacting means at a position where the process unit faces a non-printing zone on the recording medium to bring the recording medium and the process unit into the contacting state. 2. An image forming apparatus according to claim 1, characterized in that the control means (3) controls the separating and contacting means (6) so that the process units (20) assume the contact state in an order starting from the process unit provided on an upstream side with respect to a transport direction of the recording medium. 3. Image forming apparatus according to claim 1 or 2, characterized in that the control means (3) controls the separating and contacting means (6) so that a process unit (20) in an OFF state assumes the separating state with respect to the recording medium. 4. An image forming apparatus according to any one of claims 1 to 3, characterized in that each of the process units (20) includes a photoconductive body (28) and a transfer body (24) which presses the recording medium against the photoconductive body, and further comprising: means for starting at least one of the rotation drive and the discharge of the transfer body before the separating and contacting means controls the photoconductive body to assume the contact state with respect to the recording medium. 5. An image forming apparatus according to any one of claims 1 to 4, characterized in that the recording medium is a continuous recording paper having a perforation, and the non-printing zone contains the perforation. 6. An image forming apparatus according to any one of claims 1 to 5, characterized in that the control means (3) includes: means for controlling the process units to record marks for confirming the non-printing zone on the recording medium; and means for detecting the non-printing zone on the recording medium by detecting the marks. 7. An image forming apparatus according to any one of claims 1 to 6, characterized in that said process units (20) include a first process unit for recording an image with respect to a first side of the recording medium, and a second process unit for recording an image with respect to a second side opposite to the first side of the recording medium, each process unit having a developing agent and the developing agent of the first processing unit on the recording medium has a charge with a polarity opposite to the charge of the developing agent of the second processing unit on the recording medium when the developing agent of the first processing unit on the recording medium reaches the second processing unit. 8. An image forming apparatus according to claim 7, characterized in that it further comprises: charging means (31, 32) for reversing the polarity of the charge of the developing agent of the first processing unit on the recording medium before the developing agent of the first processing unit on the recording medium reaches the second processing unit. 9. An image forming apparatus according to any one of claims 1 to 8, characterized in that at least one of the process units (20) includes: a photoconductive body (28); a transfer body (24) which presses the recording medium against the photoconductive body; a guide roller (35, 37) which is provided in the vicinity of the transfer body and on an upstream side thereof with respect to a transport direction of the recording medium and guides a non-printing portion of the recording medium; and means for controlling the recording medium and the guide roller to approximately the same speed in the vicinity of the transfer body. 10. An image forming apparatus according to any one of claims 1 to 9, characterized in that it further comprises: a single fixing means (7) provided on a downstream side of all the process units (20) with respect to a transport direction of the recording medium is used to fix the images on the recording medium. 11. An image forming apparatus according to any one of claims 1 to 10, characterized in that said process units (20) include a first process unit which records an image with respect to a first side of the recording medium, and a second process unit which records an image on a second side opposite to the first side of the recording medium, which first and second process units have the same construction including a photoconductive body (28) and a developing body (23A, 23B), the photoconductive body and the developing body of the first process unit rotating in the same direction and the photoconductive body and the developing body of the second process unit rotating in opposite directions. 12. Image forming apparatus according to claim 11, characterized in that the developing body (23A) of the first process unit and the developing body (23B) of the second process unit rotate at mutually different rotation speeds. 13. An image forming apparatus according to claim 11 or 12, characterized in that the second process unit contains a developing agent having a toner content that is larger than a toner content of a developing agent contained in the first process unit. 14. An image forming apparatus according to any one of claims 11 to 13, characterized in that the first processing unit comprises a developing agent having a carrier grain diameter which differs from a carrier grain diameter of a developing agent of the second processing unit.